4.2 Mendelian Genetics

Why This Matters

Mendelian genetics is the calculation core of Unit C. Diploma questions ask you to predict ratios, read pedigrees, and grid in probabilities as numerical responses. Master the vocabulary first: a gene is a unit of heredity; an allele is a version of a gene; genotype is the allele combination (e.g. Aa); phenotype is the observable trait. Homozygous means two identical alleles (AA or aa); heterozygous means two different alleles (Aa).

Mendel's Two Laws

Gregor Mendel's pea-plant experiments produced two foundational laws:

• Law of Segregation — the two alleles for each trait separate during gamete formation, so every gamete receives exactly one allele. Fertilization restores the pair.
• Law of Independent Assortment — alleles for different traits are distributed to gametes independently of one another (true for genes on separate chromosomes).

A dominant allele (capital letter) masks a recessive allele (lowercase) in a heterozygote. A recessive phenotype appears only when an individual is homozygous recessive (aa). The P generation is the parents; F1 is the first filial (offspring) generation; F2 results from crossing F1 individuals. A test cross with a homozygous recessive (aa) partner reveals whether a dominant-looking individual is AA or Aa.

Worked Monohybrid Cross (3:1)

Cross two heterozygous tall pea plants, where T (tall) is dominant over t (short): Tt x Tt.

Each parent makes gametes T and t. The Punnett square:

Genotypes: 1 TT : 2 Tt : 1 tt. Phenotypes: 3 tall : 1 short — the classic 3:1 ratio. The probability of a short (tt) offspring is 1/4 (25%), and the chance of at least one dominant allele (tall) is 3/4 (75%).

Dihybrid Cross (9:3:3:1)

When two traits are tracked at once, a cross of two double heterozygotes (AaBb x AaBb) follows independent assortment. Each parent produces four gamete types: AB, Ab, aB, ab. Filling the 4x4 Punnett square gives the famous 9:3:3:1 phenotypic ratio:

• 9 show both dominant traits (A_B_)
• 3 show dominant A, recessive b (A_bb)
• 3 show recessive a, dominant B (aaB_)
• 1 shows both recessive traits (aabb)

Shortcut: the chance of double-recessive (aabb) is 1/4 x 1/4 = 1/16, matching the single "1" in the ratio. The multiplication rule is faster than drawing a 16-box square: treat each trait as a separate monohybrid cross (3/4 dominant, 1/4 recessive) and multiply the probabilities. For example, the chance of A_bb is 3/4 x 1/4 = 3/16. This rule is essential for the numerical-response items, which often ask for a single probability rather than the full ratio.

Beyond Simple Dominance

ABO blood type combines codominance and multiple alleles: I^A and I^B are codominant and each is dominant over i. So I^A I^B is type AB, I^A i is type A, and ii is type O.

Sex Linkage and Pedigrees

Females are XX; males are XY. Most sex-linked genes sit on the X chromosome. A recessive X-linked allele (written X^h) is expressed in any male who carries it, because his single X has no second allele to mask it. This is why colour blindness and hemophilia are far more common in males.

A carrier female (X^H X^h) shows the normal phenotype but can pass the allele to sons. Pedigree tips: squares = males, circles = females, filled = affected. A trait that skips generations and appears mainly in males signals X-linked recessive inheritance.

To distinguish patterns in a pedigree, ask: Do two unaffected parents have an affected child? Then the trait is recessive. Does every affected child have an affected parent? Then it is likely dominant. Are affected individuals mostly male, with no father-to-son transmission? Then suspect X-linked recessive. Working through these questions in order reliably identifies the inheritance mode on diploma pedigree questions.